Rheological studies have been found useful in several important arenas from medical diagnostics and biological sciences of cellular and tissue functions, to manufacturing of inks, paints and varnishes, and to better understand basic non-Newtonian materials such as polymers and liquid crystalline materials. However, some of these fields rely on a limited amount of material, especially in the biological fields (typically on the order of a few mL). Therefore, recent technological needs have driven a renewed interest in both shrinking the scale of rheological techniques and the amount of material needed to perform such measurements.
Classical rheological equipment can be generalized into three basic categories that utilize capillary, rotational, or falling/rolling ball techniques. While some success has been made in the shrinking the size scale of the latter two, they still rely on amounts of material on the order of a mL. Much more successful attempts have been made in shrinking the capillary methods, mainly due to the successes of micro-fabrication techniques in creating micro-fluidic channels. These methods have reported successful viscosity measurements using material volumes in the order of a μL down to about twenty nL. However these methods are typically valid for Newtonian flow regimes and low viscosities that in general limit their usage as a general Theological tool. Some improvements on this technique have allowed greater viscosity range (IcP<η<100 cP), but is still limited to Newtonian behavior. Many classical rheometers used to measure complex viscoelastic properties relied on oscillatory motion to induce shear stresses on a fluid. This technique has been applied to several micro-viscometer techniques based on resonance properties of cantilevers or piezoelectric crystals. Although many of these techniques rely on being submerged in a container of material, one method using quartz crystals could measure rheological properties of around 10 μL of material. This method, however, resonated between 5-10 MHz, resulting in very large shear rates, usually well above typical non-Newtonian behavior transitions.
In summary, although there have been numerous attempts at developing rheometers capable of measuring material properties with very small volumes, there has not yet been one flexible enough to perform generalized measurements over a wide range of material types, viscoelastic regimes, and broad temperature ranges.